& XSLT >  > Write a simple DTD that allows you to omit
>  > specific start and/or end tags within the file.
  
>  How would you do that? Isn't a dtd suposed to describe
>  the document structure of a well formed xml file?
 
Yes, but the notion of a Document Type Definition originally comes from the 
Standard Generalized Markup Language (SGML), also known as ISO 8879 (1986). 
XML is a subset of SGML [1]. 

Basically, one of the main differences between SGML DTDs and XML DTDs
is a feature called tag minimisation. This feature is not available in
XML and was introduced in SGML to keep file sizes small'ish. 

In XML, an element declaration looks like

	<!ELEMENT a        (b, c)> 

In SGML however, you have to specify whether tag minimisation may be used: 

	<!ELEMENT a    - - (b, c)> 

The two hyphens specify that both the start and the end tag must be present.
Alternatively, you can use "O" (that's not "0", but "O" as in "_o_missible")
to specify that only the start tag, only the end tag or both may be left
out. Tag minimisation is the reason why browsers do not insist that HTML 
elements such as <br> , <img> and <meta> have a corresponding end tag.

Kind regards,
	Georg


[1] To be precise, XML is a subset of ISO 8879 TC 2: "ISO/IEC JTC1/SC34
    N0029 -- Document Description and Processing Languages -- Technical
    Corrigendum 2 to ISO 8879:1986 (Annex K: WebSGML Adaptations; Annex L: 
    Declaration for XML)"

-- 
georg.rehm <at>  gmail.com                                    http://georg-re.hm
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